A search for H-chondritic chromite grains in sediments that formed immediately after the breakup of the L-chondrite parent body 470 Ma ago

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Chromites from Middle Ordovician fossil L chondrites and from matrix and shock-melt veins in Catherwood, Tenham, and Coorara L chondrites were studied using Raman spectroscopy and TEM. Raman spectra of chromites from fossil L chondrites showed similarities with chromites from matrix and shock-melt veins in the studied L chondrite falls and finds. Chromites from shock-melt veins of L chondrites show polycrystallinity, while the chromite grains in fossil L chondrites are single crystals. In addition, chromites from shock-melt veins in the studied L chondrites have high densities of planar fractures within the subgrains and many subgrains show intergrowths of chromite and xieite. Matrix chromite of Tenham has similar dislocation densities and planar fractures as a chromite from the fossil meteorite Golvsten 001 and higher dislocation densities than in chromite from the fossil meteorite Sextummen 003. Using this observation and knowing that the matrix of Tenham experienced 20-22 GPa and <1000°C, an upper limit for the P, T conditions of chromite from Golvsten 001 and Sextummen 003 can be estimated to be 20-22 GPa and 1000°C (shock stage S3-S6) and 20 GPa and 1000°C (S3-S5), respectively, and we conclude that the studied fossil meteorite chromites are from matrix.

Section: Introductionmentioning

confidence: 94%

Shocked chromites in fossil L chondrites: A Raman spectroscopy and transmission electron microscopy study

Rout

Heck

Zaluzec

et al. 2017

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“…The EC grains have narrowly defined ranges for Cr 2 O 3 (~53.0 to 62.0 wt%), FeO (~23.0 to 32.0 wt%), Al 2 O 3 (~4.5 to 8.5 wt%), MgO (~1.3 to 4.5 wt%), V 2 O 3 (~0.55 to 0.95 wt%), TiO 2 (~1.40 to 4.50 wt%; Schmitz and Häggström 2006; Schmitz 2013; Schmitz et al 2019b). The EC grains can also be divided into the three ordinary chondritic subgroups, depending on their TiO 2 content: H ≤ 2.50 wt%, L = 2.51‐3.39 wt%, and LL ≥ 3.40 (Heck et al 2016; Schmitz et al 2017). The TiO 2 content follows a Gaussian distribution, with about 10% overlap between the groups.…”

Section: Methodsmentioning

confidence: 99%

“…The average TiO 2 content of the H, L, and LL group is approximately 2.2, 2.7, and 3.4 wt%, respectively. The EC classification into subgroups can also be done with oxygen three‐isotopic analysis, but it has been shown that the TiO 2 approach is as effective as using oxygen isotopes (Heck et al 2016). The advantage of using both oxygen isotopes and TiO 2 is that uncertainties from compositional overlaps in respective approach can be resolved in greater detail.…”

Section: Methodsmentioning

confidence: 99%

The micrometeorite flux to Earth during the earliest Paleogene reconstructed in the Bottaccione section (Umbrian Apennines), Italy

Boschi

Schmitz

Martin

et al. 2020

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Based on sediment‐dispersed extraterrestrial spinel grains in the Bottaccione limestone section in Italy, we reconstructed the micrometeorite flux to Earth during the early Paleocene. From a total of 843 kg of limestone, 86 extraterrestrial spinel grains (12 grains > 63 μm, and 74 in the 32–63 μm fraction) have been recovered. Our results indicate that the micrometeorite flux was not elevated during the early Paleocene. Ordinary chondrites dominated over achondritic meteorites similar to the recent flux, but H chondrites dominated over L and LL chondrites (69%, 22%, and 9%, respectively). This H‐chondrite dominance is similar to that recorded within an enigmatic 3He anomaly (70, 27, and 3%) in the Turonian, but different from just before this 3He anomaly and in the early Cretaceous, where ratios are similar to the recent flux (~45%, 45%, and 10%). The K‐Ar isotopic ages of recently fallen H chondrites indicate a small impact event on the H‐chondrite parent body ~50 to 100 Ma ago. We tentatively suggest that this event is recorded by the Turonian 3He anomaly, resulting in an H‐chondrite dominance up to the Paleocene. Our sample spanning the 20 cm above the Cretaceous–Paleogene (K–Pg) boundary did not yield any spinel grains related to the K–Pg boundary impactor.

“…In addition, an increase in terrestrial cratering rates during the Middle and Late Ordovician is observed (Schmitz et al 2001) with possibly many if not all of the craters related to the breakup event (e.g., Schmitz et al 2011;Darlington et al 2016). Both elemental and isotopic studies of relict chromite and chrome-spinel grains from the fossil meteorites and micrometeorites and from impact rocks have confirmed that they are L chondritic and are directly related to the LCPB (Heck et al 2004(Heck et al , 2010(Heck et al , 2016Schmitz et al 2011;Schmitz 2013) with the exception of one fossil meteorite € Osterplana 065 ( € Ost 065), which was tentatively classified as winonaite-like (Schmitz et al 2014) but later reclassified as an ungrouped achondrite ). Whole-rock e 54 Cr and D 17 O values and elemental composition of chrome-spinel grains from € Ost 065 show that the meteorite is different from any previously known meteorites and is therefore ungrouped.…”

Section: Introductionmentioning

confidence: 95%

“…Both elemental and isotopic studies of relict chromite and chrome‐spinel grains from the fossil meteorites and micrometeorites and from impact rocks have confirmed that they are L chondritic and are directly related to the LCPB (Heck et al. , , ; Schmitz et al. ; Schmitz ) with the exception of one fossil meteorite Österplana 065 (Öst 065), which was tentatively classified as winonaite‐like (Schmitz et al.…”

Section: Introductionmentioning

confidence: 99%

Shock history of the fossil ungrouped achondrite Österplana 065: Raman spectroscopy and TEM of relict chrome‐spinel grains

Rout

Heck

Schmitz

2018

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Chrome‐spinel grains from the fossil ungrouped achondrite Österplana 065 (Öst 065) recovered from Middle Ordovician limestone in Sweden were studied using Raman spectroscopy and TEM. All the studied chrome‐spinel grains have a high density of planar fractures and planar features, not seen in chromites from the other L chondritic Ordovician fossil meteorites. Raman spectra of the host chrome‐spinel grain and its planar features are similar and no signatures of high‐pressure phases of chromite were found. The planar features occur along planar fractures, are enriched in ZnO, and are most probably produced due to enhanced leaching during terrestrial weathering in the marine sediment. Dislocation densities within two FIB sections prepared from two chrome‐spinel grains from Öst 065 are similar to the dislocation densities found within chromite grains from the matrix of Tenham L6 chondrite. Using this observation and taking into account the presence of significant fracturing in all the grains, we conclude that the Öst 065 chrome‐spinel grains were subjected to moderate to very strong shock corresponding to shock stages of S4–S6. This makes Öst 065 fossil achondrite the highest shocked fossil meteorite studied so far. This is consistent with the hypothesis that Öst 065 is a piece of the impactor that led to the L chondrite parent body breakup.